This invention relates to telephony and more particularly, to a ground start loop current detector. In general, loop current detectors are used to monitor the condition of a subscriber loop by sensing the level of current in the loop and to signal the central switching equipment indicating the sensed condition of the subscriber loop.
A typical subscriber loop is characterized by four conditions. The first condition is an idle state when the subscriber is on-hook and the subscriber loop is open so that virtually no d.c. current flows in the loop. The second condition is an initial busy state when the subscriber first goes off-hook, and the ring lead is grounded. The third state is a normal busy state which immediately follows the initial busy state. In the normal busy state the subscriber loop is closed, and the ground has been removed from the ring lead. And the fourth condition is the ring state when the subscriber is on-hook, the subscriber loop is open to d.c. current and an a.c. ringing signal is being transmitted on the ring lead.
For each of these conditions, the loop detector detects the level of d.c. current in the leads of the subscriber loop and provides a control signal to the central office switching equipment so that the subscriber loop is properly serviced.
The loop current detector must accurately detect these various conditions in order to determine when a change has occurred in the status of the subscriber loop. The first change or transition from the idle to the initial busy state occurs when the subscriber requests service by going off-hook, thereby grounding the ring lead of the loop.
The second transition from the initial busy state to the normal busy occurs when the detector senses the ground on the ring lead, and associated control circuitry signals the subscriber (by grounding the tip lead) that the central office is ready. The subscriber set in response removes the ground from ring lead and closes the loop circuit.
The third transition from the normal busy to the idle state occurs when a subscriber ends a call, goes on-hook and thus opens the subscriber loop.
And the fourth transition from the ringing to the normal busy state occurs when the subscriber, responding to the ringing of his set, goes off-hook and closes the subscriber loop.
In a ground start telephone system, only the ring lead of a loop is monitored during the idle condition of the loop for the presence of the ground -- thus the term "ground start". When the subscriber goes off-hook and requests service, the ring lead is grounded, and the detector senses the current flowing as a result of the grounded condition in order to initiate the transition from idle to normal busy.
The telephone industry has long been plagued by the problem of spurious a.c. currents induced into the subscriber loop (especially during the low current, high impedance idle condition) by outside instrumentalities such as power lines and other sources of electro-magnetic energy. The result of such electro-magnetic energy is to produce longitudinal (common mode) interference currents in the subscriber loop. The longitudinal currents flow in the same direction in both the tip and ring lead and thus add to the current in one lead and substract from the current in the other lead. If the longitudinal interference is sufficiently strong, these spurious currents can trigger the loop current detector and provide false indications to the central switching equipment as to the status of the subscriber loop.
Conventionally, in loop start detectors (as distinguished from ground start) differential amplifiers may be connected across the current loop in order to eliminate the effects of the longitudinal interference currents in the loop. U.S. Pat. No. 3,622,709 issued to Tjaden is illustrative of the approach of using a differential amplifier for a loop current detector.
Other approaches have been tried to eliminate longitudinal interference. U.S. Pat. No. 3,042,816 issued to Aagaard illustrates a detector that uses a transformer to cancel the effect of longitudinal current in the loop. The Aagaard patent shows a transformer having one winding connected in the tip lead circuit and a second winding connected in the ring circuit. Rather than being connected in series with the respective conductors, one of the windings parallels a battery feed resistor while the second winding serves as an input to a single ended detector. Thus, the Aagaard device does not operate in a ground start facility but, rather, requires current flow in both loop leads in order to cancel the effects of longitudinal currents.
Because in a ground start system only one lead of the subscriber loop is attached to the loop current detector during the idle condition, the conventional differential amplifier will not serve to eliminate longitudinal interference currents which are flowing in the single line being monitored, nor will the device shown in Aagaard operate connected to a single line. Insofar as applicants are aware, the prior art with its differential amplifiers and its loop current equalization circuits does not provide a solution to the unique problem of longitudinal interference currents for ground start operation when a single line of the subscriber loop is being monitored.